1. Field of the Invention
The present invention relates to a multilayer optical recording medium including a plurality of stacked recording and reading layers from which information can be reproduced by light irradiation.
2. Description of the Related Art
In the field of optical recording media, a wavelength of a laser light source is shortened and an NA of an optical system is increased in order to increase a recording density. For example, in an optical recording medium of a Blu-ray Disc (BD) standard, the wavelength of the laser is set to 405 nm, and the numerical aperture is set to 0.85 to allow recording and reading of 25 GB capacity per layer. However, efforts based on the light source and the optical system are reaching a limit, and volume recording for multiplexed recording of information in the optical axis direction is required to further increase the recording capacity. For example, in the optical recording medium of the Blu-ray Disc (BD) standard, a multilayer optical recording medium including eight recording and reading layers (see I. Ichimura et. al., Appl. Opt., 45, 1794-1803 (2006)) or six recording and reading layers (see K. Mishima et. al., Proc. of SPIE, 6282, 628201 (2006)) is proposed.